The purpose of a low dropout voltage regulator is to provide a predetermined and substantially constant output voltage to a load, over a wide temperature range, from a voltage source which may be poorly-specified or fluctuating. In typical low dropout regulators, the output voltage is regulated by controlling the current through a pass element (such as a power transistor) from the voltage source to the load.
Typically, low dropout voltage regulators incorporate the following primary elements (in addition to the pass device): (1) drive circuitry for controlling the current conducted by the pass device by adjusting drive to the pass device, (2) control circuitry for generating a reference signal, and for comparing a feedback signal (typically the output voltage or current, or portion thereof) to the reference signal to generate an error signal indicative of the difference between the output and reference; (3) a current source generator for providing currents to the circuits; (4) a bias circuit for biasing the current source generator, and (5) a startup circuit. The error signal generated by the control circuitry is coupled to the drive circuitry, in order to raise or lower as appropriate the drive current delivered to the pass device based on the feedback signal as compared to the reference signal. Raising or lowering the drive current adjusts the current delivered to the load and, consequently, regulates the output voltage to a desired value.
Low dropout voltage regulators are known in the prior art. While these circuits work well, they typically are unable to produce regulated output voltages lower than about 2.5 volts. An example of such a prior art low dropout regulator is disclosed in Dobkin et al. U.S. Pat. No. 5,274,323. A simplified block and circuit diagram of that prior art circuit is illustrated in FIG. 1.
The prior art circuit architecture of FIG. 1 forms a low dropout voltage regulator 100 capable of producing temperature compensated, regulated output voltages at output terminal 105 (V.sub.OUT) from about 2.5 volts to 15 volts. The circuit components within block 180 form a control circuit which includes a combined reference voltage generator and error amplifier circuit. The circuitry in block 180 produces an output error signal at node 165 as a function of the output (feedback) voltage developed at terminal 105. The error signal is coupled to current drive circuit 104, which in turn drives pass device 150 of voltage regulator 100. The components within block 160 form an impedance string to temperature compensate the control circuitry, to obtain a desired temperature drift of the control circuitry (typically zero to a first order) over a wide temperature range (typically, -50.degree. C. to 125.degree. C.). The control circuit is powered by current drawn from the output voltage 105, and biased by current source generator 103. Transistors 119 and 120 (and associated resistors 108 and 109) form current sources for a current mirror comprised of transistors 125 and 126. The emitter areas of transistors 125 and 126 are in a ratio of 1:10, respectively.
In operation, as the voltage at output (feedback) terminal 105 begins to rise, the currents flowing through the string of components including transistors 119, 118, 117 and 126, and resistors 109, 113 and 116, and the string comprised of resistor 108, transistor 120 and transistor 125, begin to rise. As the currents increase, the .DELTA.V.sub.BE voltage dropped across resistor 116 (this voltage being created as a consequence of the unequal emitter areas of transistors 125 and 126) causes the current ratio between transistors 125 and 126 to decrease. This causes the collector voltage of transistor 125 (the error signal) to decrease. When the voltage drop across resistor 116 reaches approximately 60 mv, the current ratio between the two transistors reaches 1:1. This is the stable operating point of the circuit at which the output voltage will be regulated. In the circuit of FIG. 1, the output voltage at terminal 105 will be regulated to 5 volts. If the output voltage tends to rise above 5 volts, additional current will flow through resistor 116 causing the voltage across the resistor to increase. This unbalances the circuit, causing the current ratio between transistors 125 and 126 to decrease and, hence, error signal at node 165 also to decrease. This causes drive circuit 104 to reduce the drive to pass device 150, which causes control circuit 180 to sink less current from the output terminal and the output voltage to decrease back towards the regulated point. On the other hand, if the output voltage tends to fall below the regulating point, the error signal 165 increases. This causes drive circuit 104 to increase the drive to pass device 150, thus causing the output voltage to increase towards the regulated voltage. Further details about the operation of the circuit of FIG. 1 are set forth in U.S. Pat. No. 5,274,323, the disclosure of which is incorporated herein by reference.
As stated above, the circuit of FIG. 1 patent is unable to produce a regulated output voltage having substantially zero temperature drift (to a first order) of less than about 2.5 volts. This minimum regulated output voltage results from the topology of circuit 180. Although impedance circuit 160 can be simply a resistor or combination of resistors, transistors and diodes or the like, chosen so that the output drop across it produces the proper desired regulation voltage, the circuit of FIG. 1 still requires at least two base-emitter junctions (of transistors 119 and 126) to be in series within the feedback loop of the control circuit between the feedback terminal and GROUND. Temperature compensation of these two transistors to cause a substantially zero temperature drift of the regulated output voltage (e.g., by appropriate choice of the temperature drift of the biasing currents produced by current source generator 103) requires that the feedback voltage (and, hence, the minimum output voltage) be set to a minimum of about twice the bandgap voltage (i.e., about 2.5 volts).
Accordingly, it would be desirable to provide an error amplifier for a control circuit that utilizes an efficient topology for the combination of a feedback input circuit and an error amplifier.
It would further be desirable to provide an error amplifier for a low dropout voltage regulator control circuit that enables the low dropout regulator to produce a regulated output voltage having a substantially zero temperature drift (first order) substantially below 2.5 volts.